(1) Field of the Invention
The present invention relates to a projected image displaying apparatus using a plurality of display devices, and further the present invention relates to a method of correcting color unevenness in a projected image displaying apparatus wherein color unevenness over the entire display image can be corrected, and to a display apparatus capable of automatically effecting chromaticity adjustment that is to be performed at a production stage, etc.
(2) Description of the Prior Art
In projected image displaying apparatuses using a plurality of display devices, it is required to strictly control characteristics of display elements in order to establish uniformity of chromaticity throughout the entire display image, but the chromaticity throughout the projected image varies position to position depending upon color rendering properties of a light source used, a color separation/composition system used for the light source and transmittance distribution of the display elements and other factors. Therefore it very difficult in a technological view point to establish such a strict control in a projected image display apparatus.
There is a method called "white-balance adjusting method" in which a video signal is composed of red, blue and green component video signals (which will hereinafter be referred to as R-signal, G-signal and B-signal, respectively), and amplitude of each component is variable so as to effect gain-control, whereby chromaticity can be adjusted roughly throughout the entire image.
Japanese Patent Application Laid-Open Sho 63 No.37,785 discloses a method of correcting unevenness of luminance and chromaticity arising in a case where an image screen is formed by arranging a plurality of liquid crystal display panels. This method comprises the steps of: measuring each liquid crystal display panel in its real mounted state on light intensity; generating data on uniformity ratio of illuminance for each liquid crystal display panel on the basis of the obtained light intensity so as to store the generated data in the memory; and executing a calculative operation based on the stored data and projected image data on each liquid crystal display panel so as to display a uniform projected image.
Now, a conventional chromaticity adjusting method will be explained with reference to FIG. 1, which is a block diagram showing a circuit for generating video signals to be provided for a typical liquid crystal display device. In this figure, .gamma.-correcting circuits 55r, 55g and 55b are constructed identically, and analog converting circuits 56r, 56g and 56b are also constructed all identically like an analog converting circuit 56 shown in FIG. 2.
A video signal S1 having an analog value is inputted to a projected image processing section 54 in which the inputted data is separated into R-signal, G-signal and B-signal. The thus separated signals pass through A/D converting circuits 54'r, 54'g and 54'b respectively in the projected image processing section 54 to be formed into quantized digital video signals S1R, S1G and S1B, which are in turn sent out to .gamma.-correcting circuits 55r, 55g and 55b.
The .gamma.-correcting circuits 55r, 55g and 55b are to perform .gamma.-correction for making compensation for voltage-transmittance characteristic of liquid crystal, and subject the inputted respective digital video signals S1R, S1G and S1B to .gamma.-correction to output digital video signals S.gamma.R, S.gamma.G and S.gamma.B to analog converting circuits 56r, 56g and 56b, respectively.
The analog converting circuits 56r, 56g and 56b convert the inputted digital video signals S.gamma.R, S.gamma.G and S.gamma.B into analog values, respectively. The thus formed analog values are sent out as video signal SR, SG and SB to a liquid crystal display device/optical converting section 57.
The liquid crystal display device/optical converting section 57, based on the inputted video signal SR, SG and SB, reproduces an image and emits light output RAY.
A chromaticity meter receives the light output RAY emitted as an image from the liquid crystal display device/optical converting section 57, and measures the chromaticity thereof to indicate the measurement. Here, for convenience of description, liquid crystal display device used is assumed to be of a normally white type, more explicitly, the liquid crystal display device is assumed to lower its transmittance as the voltage applied increases. Further assumption is that the device is driven by an alternating voltage which has a center voltage of ground level and inverts its polarity for every horizontal line.
Next, description will be made of converting procedures of signal waveforms in the analog converting circuits 56r, 56g and 56b. FIG. 2 is a circuit diagram of the analog converting circuit 56. There are three analog converting circuits 56 on the diagram shown in FIG. 1, namely, analog converting circuit 56r for processing the R-signal, analog converting circuit 56g for processing the G-signal and analog converting circuit 56b for processing the B-signal. Here, FIGS. 3A through 3F are diagrams showing waveforms of video signals for different points in 56.
As shown in FIG. 3B, a control signal S6 has a waveform taking a peak value, or voltage H for `High` level and taking another peak value, or voltage L for `Low` level, and alternating between `High` level and `Low` level, periodically. This signal is inputted into a logical circuit 62, switches SW1, SW2 and SW3, and serves as a synchronizing signal.
When the control signal S6 is at `High` level in switches SW1, SW2 and SW3, all terminals `c` are simultaneously connected with respective terminals `b` and are simultaneously disconnected with respective terminals `a`. On the other hand, when the control signal S6 is at `Low` level, all terminals `c` are simultaneously connected with respective terminals `a` and are simultaneously disconnected with respective terminals `b`.
In the switch SW1, the terminal `a` is applied with a full-scale voltage V5 that is adjusted by a variable resistance R1 whereas the terminal `b` is grounded. As previously described, the terminals `a` and `b` are alternately connected with the terminal `c` following the control signal S6 inputted to a terminal `d`. With the alternating connection, the terminal `c` generates a pulse having a peak value equal to the full-scale voltage VFS and launches the pulse as a signal S11 to a negative (-)terminal of an amplifier AMP1. The signal S11 inputted to the (-)terminal of the amplifier AMP1 is therein multiplied by (-1) to output a waveform shown in FIG. 3D.
In the switch SW2, the terminal `a` is applied with an offset voltage VOF that is adjusted by a variable resistance R2 whereas the terminal `b` is grounded. As previously described, the terminals `a` and `b` are alternately connected with the terminal `c` following the control signal S6 inputted to a terminal `d`. With the alternating connection, the terminal `c` generates a pulse having a peak value equal to the offset voltage VOF and launches the pulse as a signal S16 to a positive (+)terminal of an amplifier AMP3. In the switch SW3, the terminal `b` is applied with an offset voltage VOF that is adjusted by a variable resistance R2 whereas the terminal `a` is grounded. As previously described, the terminals `a` and `b` are alternately connected with the terminal `c` following the control signal S6 inputted to a terminal `d`. With the alternating connection, the terminal `c` generates a pulse having a peak value equal to the offset voltage VOF and launches the pulse as a signal S17 to a negative (-)terminal of the amplifier AMP3. Accordingly, these signals S16 and S17 synchronize with each other, but are logically inverted one another, or more specifically, when the signal S16 stays at `High` level, the signal S17 is at `Low` level.
The AMP3 subtracts the signal S17 inputted into the (-)terminal from the signal S16 inputted into the (+)terminal, and outputs the resultant signal S13 to a (-)terminal of an amplifier AMP2.
A digital video signal S8 corresponds to any one of the digital video signals S.gamma.R, S.gamma.G and S.gamma.B that have been quantized by and outputted from .gamma.-correcting circuit 55 r, 55g and 55b. By the way, the .gamma.-correcting circuit 55 r, 55g and 55b execute conversion of an inputted signal to form an output in accordance with a characteristic curve shown in FIG. 4C, therefore, the digital video signal S8 is supplied to a logical circuit 62 as having a waveform taking a certain constant voltage VP shown in FIG. 3A as a peak value.
The logical circuit 62 also receives a signal S6 alternately reversing at intervals of one horizontal period as having a waveform shown in FIG. 3B, and effects a logical operation between the signal S6 and the digital video signal S8, whereby the digital video signal S8 is converted into digital video signal data series D0, D1, D2, (where the digital video signal S8 is digitized to indicate waveform constituents as it is and as inverted alternately every one horizontal period) as shown in FIG. 3AA to be outputted to a D/A converter 59.
The D/A converter 59 receives at its referential voltage input terminal VREF an input of full-scale voltage VFS that has been adjusted by means of the variable resistance R1, and based on the input, converts the inputted video signal data series D0, D1, D2, . . . into a corresponding analog signal, so as to form a video signal S9 having a waveform with the full-scale voltage VFS as a peak value as shown in FIG. 3C. The thus generated signal S9 is outputted from an output terminal VOUT to a (+)terminal of the amplifier AMP1.
The amplifier AMP1 subtracts the signal S11 from the signal S9, and the resultant is outputted as a signal S12 to a (+)terminal of the amplifier AMP2. The signal S9 has a waveform shown in FIG. 3C and the signal S11 multiplied by (-1) takes a waveform shown in FIG. 3D. Accordingly, the signal S12 will have a waveform shown in FIG. 3CC formed by the sum of the waveform shown in FIG. 3C and the waveform shown in FIG. 3D.
The amplifier AMP2 subtracts the signal S13 from the signal S12, and the resultant is outputted as a video signal S14 to the liquid crystal display device/optical converting section 57. This video signal S14 corresponds to any one of the video signals SR, SG and SB. The signal S12 has a waveform shown in FIG. 3CC, and the signal S13 multiplied by (-1) takes a waveform shown in FIG. 3E. Therefore, the signal S14 will have a waveform shown in FIG. 3F formed by the sum of the waveform shown in FIG. 3E and the waveform shown in FIG. 3E.
As mentioned above, it is difficult for the conventional white-balance adjustment to inhibit the color unevenness in the image occurring due to the dispersion of individual device elements on the display image. On the other hand, the object of Japanese Patent Application Laid-Open Sho 63 No.37,785 cited above is to make correction between liquid crystal display panels arranged, therefore no irregularity or unevenness within the image cannot be corrected.
Besides, in accordance with the configuration shown in FIG. 1, a display apparatus is constructed by three liquid crystal display devices for red, green and blue (which will hereinafter be referred to as R, G and B, respectively, and the three color, red, green and blue will be referred to as RGB.) having three respective circuits one for each. Therefore, these circuits are to involve dispersion in their circuit constants, and the three liquid crystal display devices must also have dispersion in voltage-transmittance characteristics. Such dispersion causes disadvantageous effects such as coloring an area that should be displayed in achromatic color like black and white. This disadvantage necessitates individual adjustment of the offset voltage VOF and the full-scale voltage VFS for each circuit.
In the prior art, this adjustment required the following procedures. Initially, data-input is made of a predetermined projected image such as, for example, a blank pattern for making pure white display on an entire image screen. Then, while the projected output generated by the optical conversion from the read data image should be measured using a chromaticity meter 58, the amplitude of the signals provided for the liquid crystal devices and the bias, more clearly, the full-scale voltage VFS and the offset voltage VOF for each device should be controlled by varying the variable resistance R1 for governing the full-scale voltage VFS and the resistance of the variable resistor R2 for governing the offset voltage VOF for each color block, so as to adjust the color-balance such as black and white.
However, this procedure requires an extremely fine adjustment and to make matters worse, it is very difficult to distinguish what resistors of which color block should be adjusted in what degree from the observed deviation of chromaticity. Therefore this method required even a skilled operator to take a very long time for the adjustment, posing a difficulty in production.